How do find all instances of a mesh via Python?
For all meshes and their instances, you can loop over the mesh objects and collect objects that correspond to the same mesh using a dictionary:
import bpy
from collections import defaultdict

result = defaultdict(list)

for obj in [o for o in bpy.data.objects if o.type == 'MESH']:
    result[obj.data].append(obj)
    
for mesh, objects in result.items():
    print( &quot;Mesh &quot; + mesh.name + &quot; has &quot; + &quot;, &quot;.join(o.name for o in objects) + &quot; object(s) for instance(s)&quot;)

Or, to find instances of one mesh:
import bpy

obj = bpy.data.objects['Cube']

instances = [o for o in bpy.data.objects if o.data == obj.data]

print(instances)